I’d say start with Facebook then work up to real life. Write on his wall, using something in his profile. (“Hey you like Pee-Wee’s Big Adventure? It’s my favorite movie ever. Watching it tonight.”) Or something about school or somethign, even a joke. If you can get a friendly banter going on facebook, it’ll be much easier in real life.
Personally, I would send him a facebook message mentioning my crush on him, but I didn’t have that confidence in high school. If you can muster it up, it’ll get the job done much better.
To be exact, I would write something on his wall, like “Hey, did you take chem last year? I’m doing homework and I left my periodic table at school and I was hoping you could help with something.” Then I would send him a message saying ” ‘Hey, did you take chem last year?’ = ‘I kind of have a big crush on you.’ Just wanted to let you know.”
Be brave! If you really want him you’ll have to take action! I guarantee he’ll be impressed that you’re being forward, and I also promise you that if it doesn’t work out it won’t be mortifying.